1. Field of the Invention
The present invention relates to an E-O probe applied to a voltage detecting apparatus which measures without electrical contact a voltage of a measured object, by detecting a strength of an electrical field generated from the measured object, and particularly to an E-O probe with a structure which enables its spatial resolution to be improved.
2. Related Background Art
There are some apparatus known as a voltage detecting apparatus which measures without electrical contact a voltage of a measured object, making use of such an E-O Probe. For example, see U.S. Pat. No. 4,618,819. In those apparatus, E-O probe is made of an electro-optic material such as LiTaO.sub.3 which has an electro-optic effect causing an index of refraction to vary in response to an intensity of an electrical field generated from a measured object.
Referring to FIG. 7, the principles of such a voltage detecting apparatus are described hereunder. The E-O probe includes an electro-optic material 1 formed as a quadrangular pyramid or the like with a flat face at the top end, and a reflecting mirror 2 attached to the flat face with the reflecting face facing the flat face, and during measuring the mirror 2 is kept in the proximity of the measured object 3. An incident beam h.nu.1 polarized as linearly polarized, circularly polarized or ovally polarized light enters the electro-optic material 1, being directed to the reflecting mirror 2, from the other side of the material 1. A light h.nu.2 reflected by the mirror 2 is converted to a light intensity by a polarization analyzing means and further the light intensity is measured by a photo-electric converting element. Since the electro-optic material 1 has a characteristic that an index of refraction varies in response to the intensity of an electrical field caused by a voltage at the measured object 3, the polarization status of the reflected light h.nu.2 is changed from the incident beam h.nu.1, and when the reflected light h.nu.2 is measured with a polarizing beam splitter and a photo-detector, a voltage at the measured object 3 can be detected. Where, the photo-detector is of photo-electric effect such as a photomultiplier tube, a high responsivity charge coupled device and the like.
As mentioned above, since a voltage of a measured object is measured optically based on an intensity of an electrical field from a measured object, a non-contact measurement is possible. This is especially efficient in such a case as measuring a signal voltage at each wire, electric line, electrode or element in a semiconductor chip and determining an operation characteristic of the semiconductor chip.
Recent advances in high-density device technologies of semiconductor integration circuits are requiring an extremely narrower space between wires of the circuits. And further improvement in spatial resolution is desired so as to enable a signal voltage propagating in each wire to be measured with high precision without being affected by an electrical field from each other wire.
Where .lambda. is a wavelength of an incident beam h.nu.1 emitted by a semiconductor laser or the like, NA is a numerical aperture of an objective lens to be used for focusing and a.sub.0 is a diameter of the incident beam h.nu.1 reflected by a reflecting mirror 2, the formula a.sub.0 =(2.times..lambda.)/(.pi..times.NA) is met. It means the shorter the wavelength of an incident beam h.nu.1 is, the smaller the diameter a.sub.0 is, and consequently the higher spatial resolution is attained. For example, in a case of .lambda.=780 nm and NA=0.4, a.sub.0 is 1.2 .mu.m, and a voltage at a wire in a circuit with a close space of 1.2 .mu.m or larger between wires on a semiconductor chip can be measured without any contact. Actually, this is a theoretical minimum beam diameter to be calculated based on a wavelength of an applied light and it is difficult to realize the minimum value due to aberration and wave front distortion of optical elements such as lens.
As described in the formula above, the highest spatial resolution is now limited to about 1 .mu.m which can not sufficiently satisfy the requirements in response to the recent advance in fine semiconductor processing technologies.
Further, no attention has been paid to the thickness of the electro-optic material 1 (the height of the quadrangular pyramid), and a relatively thick material was used. Under the circumstances, an overall polarization status of the electro-optic material 1 varies even when the tip portion of the electro-optic material 1 is not just above the place to be measured of the measured object 3, since the other part than the tip portion is also placed under the influence of the electrical field from the measured object 3 as illustrated in FIG. 8. It results in a difficulty to achieve a high spatial resolution. With a thick electro-optic material 1, a polarization status would vary in response to an electrical field from the measured object 3 not only in the tip portion of the electro-optic material of E-O probe but also in the body portion of the electro-optic material 1, even when the tip portion is not just above the measured point on the measured object 3.